In electrophotographic printing, a pattern of electrostatic charges corresponding to a print image is applied to an optical photoreceptor (OPR). Toner is applied to the OPR and that toner which is retained as a result of not being repelled by electrostatic charges is used to form the print image. The print image is then transferred to a print media (usually paper).
The OPR may use either visible spectrum light or optical energy outside the visible light spectrum. In the preferred embodiment, it is anticipated that infrared light will be used, but the OPR as described in connection with this invention is intended to mean any photoreceptor which responds to radiated energy.
In the case of laser printing, the surface of the OPR is charged, and the image is created by selectively exposing the charged surface to light (the laser). The exposure to light results in the depletion of surface charges.
The OPR is usually a continuous surface such as a drum or belt, and is used repeatedly for subsequent sequential print operations. The toner applied to the OPR during each print operation must be removed from the OPR, except in the pattern of the print image, before transfer of the print image from the OPR.
The toner is commonly delivered either as a powder or as a liquid emulsion, referred to in the art as, "liquid toner." Liquid toner consists of solid toner particles suspended in a liquid carrier, and is sometimes also referred to as liquid emulsion toner. The image provided by liquid toner solution is provided by the solid toner particles, and liquid carrier is either recycled within the unit or is lost.
In the case of a liquid toner, the toner is applied to the charged OPR, and excess emulsion is allowed to drain off of the OPR. This leaves the OPR with a coating of emulsion. At locations where the OPR charge is depleted (typically by the laser light), toner particles from the emulsion are concentrated. This concentration of toner particles is the result of the particles not being repelled by electrostatic charge. The concentration of particles is referred to as developing the image. Excess toner which is not retained, as a result of the electrostatic charge repelling the particles, is not part of the image and may be referred to as "non-image toner".
In one arrangement, toner is applied to the OPR by a developer roller. The developer roller retains non-image toner which is repelled by the OPR. Some additional non-image toner may also be retained by the OPR, which may be subsequently removed. As a result of the OPR attracting particles, the non-image toner not retained by the developer roller is mostly superfluous carrier liquid, and tends to have a reduced solid component over that retained by the developer roller.
At locations on the OPR which are charged, toner particles are not retained by the OPR (the non-image toner). In the case of liquid toner, solid toner particles in the emulsion which are not retained by the developer roller as an image are subsequently removed. The action of applying of the liquid toner results in the toner particles which are not retained by the OPR being retained by the developer roller and potentially interfering with the proper subsequent transfer of toner to the OPR. In the present design, the remaining liquid from the liquid toner solution which is dispensed at a developer station and not removed by the development process is removed at an extraction station. At the extraction station, the non-image toner fluid which remains on the OPR has a substantially reduced solid toner content; that is, substantially devoid of toner particles.
The concentration of solid particles in liquid toner solution, as applied to the OPR, is typically in the range of 2% to 9%, as is the concentration of the solution in the toner supply. The toner being applied onto print media has approximately 97% solid concentration. (Percentages would be by volume and weight, since it is important that the specific gravities of the components be approximately equal.) This can be compared to starting with cloudy water at the development station, and ending up with visibly dry mud at the print media surface. In increasing these concentrations, it sometimes becomes difficult to transport and remove the emulsion.
After the OPR is coated, an additional amount of liquid from the liquid emulsion is removed by passing the coated OPR through a nib. The remaining material from the toner emulsion is then further dried and transferred to the print media.
Printer resolution is primarily a function of the size of the optical image generator (the laser or optics) and toner particle size. Typical solid toner laser printers, such as the Assignee's Laser Jet 4L.TM. printer, use dry toner having a toner particle size of approximately 5-7 microns (.mu.M) diameter. It is desired to reduce particle size to approximately 0.2 .mu.M by the use of liquid toner solution.
The present invention relates to a system for handling toner in which particle size is decreased. While liquid toner solution is used to facilitate smaller particle size, the smaller particle size imposes additional requirements on the toner delivery system. In particular, it is important to provide a means for evenly dispersing the emulsion and maintaining the emulsion at a consistent state.
The liquid toner solution must also be provided to the developer in a well mixed, homogenous, state. If return fluid from the developer station or elsewhere is mixed with toner from a supply reservoir, the toner from the supply reservoir will be homogeneous. This mixed return fluid makes it easier to maintain homogeneity of the toner after mixing with return fluid. Additionally, the toner from the supply reservoir typically has been standing for long time periods, and so may be subject to particles clumping, settling or other non-homogenizing effects. The toner which is coated onto the developer roller is likely to consist mostly of the toner supplied directly from the supply reservoir, so thorough homogenization of this toner results in an overall better quality printed image. Thorough homogenization of the toner from the supply reservoir also results in better control of particle separation at the developer and extraction stations, and therefore more precise development.
In our preferred system for delivering liquid toner solution, a developer roller is used to apply emulsion to the OPR. The emulsion is supplied to the developer roller which in turn transfers the emulsion to the OPR. A substantial amount of emulsion is removed from the OPR by virtue of the fact that the emulsion is electrostatically repelled from the OPR where the OPR is charged. Additional solids-depleted emulsion is removed at a nib pressure roller station.
Excess fluid on the OPR has a tendency to spread outwardly toward the edges of the OPR. This results in excess fluid along the edges where the fluid tends to wrap around at the edge of the OPR at the extraction nib roller due to capillary effects. It would be advantageous if this wrap around could be minimized or eliminated.
The developer roller has a tendency to be charged to a potential which is less than that of the charged portion of the OPR, but greater than ground potential. This tends to allow the developer roller to attract a pattern of toner which is essentially a negative image of that applied to the OPR. Unlike dry toner, liquid toner is electrically neutral; i.e., zero net charge, but consists of an equal amount of positively charged and negatively charged species. The core of the developer roller is maintained at a bias voltage, but liquid toner solution can provide an insulating film on the developer roller, insulating the surface of the roller. Furthermore, the negative image pattern attracted by the developer roller is separated from the emulsion and tends to remain on the developer roller.
It is advantageous to recover the non-image toner fluid in order to keep carrier consumption to a minimum, and hence keep toner consumption to a minimum. This also reduces the need to dispose of waste products. Therefore, it is desired to return as much of the non-image toner fluid as possible to the fluid reservoir without adversely affecting the quality of the toner fluid supplied from the reservoir. When applying the emulsion, that emulsion which is returned to the developer roller must be removed and properly mixed in order that the developing process not be adversely affected by the toner emulsion coating the developer roller.
In order to maintain a high print quality, it is desired to keep the surface of the OPR dry, except during the application of toner. If the OPR surface is wet or "soupy," the image will be diffused as the charge on the OPR surface is depleted. It is therefore desired to provide a system in which image drying is facilitated by extracting as much liquid component as possible from the surface of the OPR.
In general, it is desired to provide a system for delivering liquid toner which facilitates the enhancement of image resolution. It is also desired to permit the use of toner having smaller particle size, with a minimum of toner waste and with a minimum of mechanical complexity.